Update docs to encourage contributors to self-label PRs with the plug…

…in they affect

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,6 +11,7 @@
 Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes (`[x]`). This will ensure a smooth and quick review process.
 
 - [ ] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
+- [ ] If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
 - [ ] My PR includes unit or integration tests for *all* changed/updated/fixed behaviors (See [Contributor Guide]).
 - [ ] All existing and new tests are passing.
 - [ ] I updated/added relevant documentation (doc comments with `///`).

CONTRIBUTING.md

@@ -6,7 +6,6 @@ _See also: [Flutter's code of conduct](https://flutter.io/design-principles/#cod
 
 ## Things you will need
 
-
  * Linux, Mac OS X, or Windows.
  * git (used for source version control).
  * An ssh client (used to authenticate with GitHub).
@@ -89,6 +88,9 @@ To send us a pull request:
 
 Please make sure all your checkins have detailed commit messages explaining the patch.
 
+For pull requests that affect only one Flutterfire plugin, use a title that starts
+with the name of the plugin in brackets (e.g. [cloud_firestore]).
+
 Plugins tests are run automatically on contributions using Cirrus CI. However, due to
 cost constraints, pull requests from non-committers may not run all the tests
 automatically.